Oxygen sensors have previously been described, for example, in German patent disclosure document DE-OS 29 42 494 in which a solid electrolyte tube is used which has a closed bottom wall at the measuring end of the tube. The surface of the solid electrolyte tube has a counterelectrode at the inside thereof, in layer form. The outer surface of the solid electrolyte tube has a measuring electrode which is porous, and gas permeable. The measuring electrode is entirely covered with a solid porous layer which has a predetermined diffusion resistance for oxygen molecules. Such a diffusion resistance layer may be made of aluminum oxide or magnesium spinel--see, also, the referenced U.S. Pat. No. 4,356,065, Dietz. The measuring electrode and the counterelectrode have a direct voltage applied thereto, in the order of about 1 volt. The measuring portion of the solid electrolyte tube can be heated by an electrical resistance heater.
Manufacturing the diffusion resistance layer, that is, the resistance layer for the oxygen molecules, is complex and a high-precision process; the porous layer must have precisely defined characteristics, requiring highly precise and low tolerance manufacturing steps so that the pores of the diffusion resistance porous layer will be neither too small, nor too large, thus interfering, on the one hand, with diffusion of oxygen molecules or permitting excess oxygen molecules to reach the measuring electrode. The basic concept consists in limiting the quantity of the oxygen which reaches the electrode to such values that, after the electrodes have a voltage thereacross, an oxygen molecule which migrates to, and reaches, the measuring electrode is immediately converted by electrode reaction. The current which will flow between the electrodes of the sensor will have a value determined solely by the amount of oxygen molecules reaching the electrode after diffusion.
The German disclosure document DE-OS 29 42 494 also describes the heater structure. In accordance with this disclosure, the heater structure is secured to a protective tube, or integrated therewith, surrounding the solid electrolyte tube. It has also been proposed to provide a heater for the solid electrolyte tube by locating the heater in the form of a heater rod extending within the interior of the hollow solid electrolyte tube--see, for example, the referenced German utility model publication No. 81 01,584.
U.S. Pat. No. 4,292,158, Muller et al. assigned to the assignee of the present application, describes manufacture of the diffusion resistance portion or element in form of gas permeable channels or ducts rather than in form of a porous coating. This is a suitable arrangement--although, also, requiring high precision manufacturing technology.